Type IA or insulin dependent diabetes (T1D) results from of an autoimmune process that destroys insulin producing beta cells in the pancreatic islets. Clinical trials demonstrate that targeting the adaptive immune system can preserve 2 cell function, but a major road block to intervention in T1D is limited access to critical early stages of the disorder. The goal of this project is to validate the position of a novel CD4+ T cell subset, T follicular helper cells (TFH) as critical early mediators of the breach of tolerance in T1D. Recent advances in vaccine biology reveal that high affinity IgG antibodies require the action of TFH. Accordingly, we propose the high affinity autoantibodies in T1D reflect the actions of TFH at a critical early checkpoint in the disease. To investigate TFH differentiation and function in the progression of T1D in NOD mice, we developed a new model that uses a targeted (knock-in) anti-insulin VH gene to detect TFH driven class switch recombination (IgG) in T1D. This model will be tested in Specific Aims that 1) identify the sites and stages in T1D when TFH initiate loss of immune tolerance, and 2) determine the contribution of regulatory T cells to the genesis of TFH in T1D. The model will validate a new early checkpoint in T1D progression and will provide better targets for early diagnosis and intervention in the future. .